The present invention relates to an ink jet printer, and more particularly to an ink jet printer having a plurality of print heads.
The following techniques are conventionally known as the ink jet printers of the aforementioned kind. Technique disclosed in (J01) the Unexamined Japanese Patent Application Publication No. Hei. 2-198850:
This publication shows a carriage on which at least two ink jet heads can be mounted. In its embodiment, a print head for ejecting ink of two or four colors is incorporated in one ink jet head.
Technique disclosed in (J02) the Unexamined Japanese Patent Application Publication No. Hei. 4-353467:
In this publication, a technique for increasing the density of black characters by setting the size of droplets of black ink to a large size.
Problem of (J01)
With the technique of (J01) described above, since the size and weight of the ink droplet vary for each color, there has been a problem in that color offset occurs in a case where the distance between the paper and the print head which ejects ink at the same timing is fixed.
Problem of (J02)
With the technique of (J02) described above, if the size of the ink droplet is enlarged with respect to the ink of the other color(s), the kinetic energy generally increases and the ink droplet ejection velocity increases by the portion in which the weight of the ink droplet increases. Accordingly, since the ink droplet ejection velocities of the black ink and the color ink do not coincide, there has been a problem in that color offset is liable to occur.
As another method, it is conceivable to increase the density of black characters by increasing the density of the dye of the black ink or changing the composition of a solvent. In this case, the ink droplet ejection velocity changes in many cases. Therefore, since the ink droplet ejection velocities of the black ink and the color ink do not coincide, there has been a problem in that color offset is liable to occur.
To prevent the color offset, it is necessary to optimize the printing timing, the drive voltage, the pulse width, values of resistance of heating elements, and the like.
As still another problem, there has been a drawback in that if the temperature of the print head rises, the size of the ink droplet becomes large, and the ink droplet ejection velocity also increases. In the past experience, a relationship that the portion of an increase in the ejection velocity=about +1%/.degree.C. has been obtained. Accordingly, there has been a problem in that even if the printing timing is set in such a manner that the color offset becomes minimal at a certain temperature, the color offset becomes large with an increase in the temperature of the print head.